The new generation of metallocene catalyzed polymers is expected to have profound effects on the polyolefin industry. Polyolefin resins produced by metallocene catalysts in general have a number of improved properties as compared to conventional polymers produced with Ziegler-Natta catalysts. As more and more product manufacturers begin utilizing metallocene produced polymers, various areas for polymer improvement will become apparent. Currently, polymer producers may choose from a variety of techniques to manipulate the polymer such as post reactor polymer degradation, blending, change of catalyst, use of additives and varying the polymerization conditions.
Metallocene catalyzed propylene polymers are among the newest metallocene produced polymers now commercially available. One major product area for these new polymers is in nonwoven fabrics. The metallocene produced propylene polymers have a number of advantages over conventional polymers. For example, these polymers have a narrow molecular weight distribution which enable significantly higher spinning speeds and finer denier fibers. In addition, due to their unique molecular architecture, these polymers have an extractable level that is less than one third that of conventional polymers.
In an effort to improve the strength of fabrics made from metallocene produced polypropylene, the present inventors sought to manipulate the polymer tacticity distribution. It was hypothesized that broadening the tacticity distribution of the propylene polymer would result in a broadened bonding window and higher strength fabric. The present inventors have found that the tacticity distribution of isotactic polypropylene can be broadened by making the polymer with at least two different metallocenes, provided that two or more metallocenes are chosen that produce polymer having different tacticities.
It is known in the art to use two metallocenes to make polymer having broadened molecular weight distribution. EP 0 310 734, for example discloses the use of at least two different metallocene catalysts which are chiral and stereorigid in combination with an aluminum compound to produce polymer having a broad molecular weight distribution. EP 645 401 also describes the use of at least two stereorigid metallocenes, of which at least one has substituted indenyl ligands. These catalysts produce high molecular weight isotactic polypropylene having a broad molecular weight distribution.
WO 94/28034 describes supported metallocene catalyst systems containing one metallocene. These metallocenes have high activity and are capable of producing high molecular weight isotactic polypropylene without fouling the reactor. The catalyst system is prepared by premixing the metallocene and alumoxane activator then combining the reaction product with support material.
Thus the prior art teaches that high molecular weight, isotactic polypropylene may be produced with supported metallocene catalyst systems. The prior art further teaches the preparation of high molecular weight istotactic polypropylene having a broad molecular weight distribution using two or more stereorigid metallocenes, of which at least one has substituted indenyl ligands. The prior art, however, does not address the problems that may occur in nonwoven fabrics prepared with polymers having narrow tacticity distribution.
Nonwoven polypropylene fabrics are well known. Nonwoven fabrics or webs may be formed by a variety of processes such as melt blowing, spunbonding, film aperturing, and staple fiber carding. They are used to make many types of products including garments such as work wear, medical garb, and absorbent articles such as diapers. Processes for making nonwoven fabrics generally involve meltspinning thermoplastic material, drawing the filaments using pneumatic means, and collecting the drawn filaments randomly on a conveyor belt and heating the web to cause the filaments to soften and fuse together. The following patents disclose various meltspinning techniques and are herein incorporated by reference: U.S. Pat. Nos. 4,692,618; 4,692,106; 4,405,297; 4,340,563; 3,692,618; 3,338,992, 3,341,394; 3,502,538; 3,502,763; 3,909,009; and 3,542,615. To the extent that these references are directed to polypropylene fabrics, they are limited to polypropylenes produced with traditional Ziegler-Natta catalysts. The fabrics made from these polymers, therefore, do not share the same attributes as fabrics prepared with single sited, metallocene produced polypropylene.
EP 600 461 and WO 9 482 219 disclose nonwoven fabrics prepared with polypropylene produced with catalyst systems having a single metallocene. The isotactic polypropylene produced from these catalyst systems, therefore, has very narrow molecular weight and tacticity distributions. The fabrics made with metallocene produced polypropylene exhibit improvements in properties such as improved web uniformity, finer fibers and better coverage which typically lead to improved softness and liquid barrier capability when compared to fabrics made with Ziegler-Natta produced polypropylene.
The present inventors have found that polypropylene nonwoven fabric strength is improved by preparing the fabric with polymer comprising polypropylene produced with a catalyst system having at least two metallocenes.